1. Field of the Invention
This invention relates to a light-emitting device which uses, as a source of light, an element (hereinafter referred to as xe2x80x9corganic EL elementxe2x80x9d) comprising a layer (hereinafter referred to as xe2x80x9corganic EL filmxe2x80x9d) containing an organic compound capable of obtaining luminescence (electroluminescence, hereinafter referred to as xe2x80x9cELxe2x80x9d) that takes place upon the application of an electric field, an anode layer and a cathode layer. The EL in the organic compound can be divided into the one that emits light (fluorescent light) when a singlet excited state returns back to a ground state and the one that emits light (phosphorescent light) when a triplet excited state returns back to the ground state. This invention is particularly concerned with a light-emitting device using an organic compound capable of generating phosphorescent light as EL. In this specification, the light-emitting device stands for a picture display device or a light-emitting device using an organic EL element as a light-emitting element. A module in which a TAB (tape automated bonding) tape or a TCP (tape carrier package) is mounted on the organic EL element, a module in which a printed wiring board is provided at an end of the TAB tape or the TCP and a module in which an IC (integrated circuit) is mounted on the organic EL element by the COG (chip on glass) system all pertain to the light-emitting devices.
2. Prior Art
The organic EL element is the one that emits light upon the application of an electric field, and is drawing attention as a flat panel display element of the next generation owing to its properties such as reduced weight, operation on a low DC voltage and high-speed response. Besides, the organic EL element emits light by itself offering a wide visual angle, from which it is expected that the organic EL element can be effectively utilized as a display screen for portable devices.
It is said that the organic EL element has a light emitting mechanism in which the electrons injected through a cathode recombine with the holes injected through an anode to form molecules in an excited state (hereinafter referred to as xe2x80x9cmolecular excitersxe2x80x9d), and energy is released when the molecular exciters return back to the ground state to emit light. The excited state can take the form of a singlet state (S*) and a triplet state (T*), and it has been considered that the statistic ratio of formation is S*:T*=1:3 (literature 1: Tetsuo Tsutsui, xe2x80x9cAcademy of Applied Physics, Organic Molecules/Division of Bioelectronics/Text of Third Lecturexe2x80x9d, p. 31, 1993).
When a general organic compound is maintained at room temperature, however, emission of light (phosphorescent light) from the triplet excited state (T*) is not observed. This also holds even for the organic EL element and, usually, the emission of light (fluorescent light) from the singlet excited state (S*) only is observed. Therefore, the theoretical limit of internal quantum efficiency (ratio of photons that are generated to the carriers that are injected) of the organic EL element has been considered to be 25% based on a ground that S*:T*=1:3.
Light that is emitted is not all released to the outside of the light-emitting device, and part of light is not recovered due to the materials (organic EL film, electrodes) constituting the organic EL element and due to refractive index specific to the substrate material. The ratio of the emitted light taken out of the light-emitting device is called light recovery efficiency. When the organic EL element is provided on the glass substrate, it has been said that the recovery efficiency is about 20%.
Because of the above reasons, even if the injected carriers have all formed exciters, it has been said that the theoretical limit of the ratio (hereinafter referred to as xe2x80x9cexternal quantum efficiencyxe2x80x9d) of the photons that can be finally taken out of the light-emitting device to the number of the injected carriers, is 25%xc3x9720%=5%. That is, even if the carriers are all recombined, only 5% of them is recovered as light.
In recent years, however, there have been successively announced organic EL elements capable of converting energy (hereinafter referred to as xe2x80x9ctriplet excited energyxe2x80x9d) released at the time when the triplet excited state returns back to the ground state into light, and their high light-emitting efficiencies are now drawing attention (literature 2: D. F. O""Brien, M. A. Baldo, M. E. Thompson and S. R. Forrest, xe2x80x9cImproved Energy Transfer in Electrophosphorescent Devicesxe2x80x9d, Applied Physics Letters, Vol. 74, No. 3, 442-444, 1999)(literature 3: Tetsuo Tsutsui, Moon-Jae Yang, Masayuki Yahiro, Kenji Nakamura, Teruichi Watanabe, Taishi Tsuji, Yoshinori Fukuda, Takeo Wakimoto and Satoshi Miyaguchi, xe2x80x9cHigh Quantum Efficiency in Organic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Centerxe2x80x9d, Japanese Journal of Applied Physics, Vol. 38, L1502-L1504, 1999).
The literature 2 uses a metal complex with platinum as a central metal (hereinafter referred to as xe2x80x9cplatinum complexxe2x80x9d) and the literature 3 uses a metal complex with iridium as a central metal (hereinafter referred to as xe2x80x9ciridium complexxe2x80x9d). It can be said that either metal complex has a feature of introducing an element in the third transition system as a central metal. Some of them easily surpass the above-mentioned theoretical limit of 5% of the external quantum efficiency.
By alternatingly laminating a layer of the iridium complex and a layer of DCM2 which is a known fluorescent coloring matter, further, the triplet excited energy formed by the iridium complex can be migrated into the DCM2 contributing to emitting light from the DCM2 (literature 4: M. A. Baldo, M. E. Thompson and S. R. Forrest, xe2x80x9cHigh-Efficiency Fluorescent Organic Light-Emitting Devices using a Phosphorescent Sensitizerxe2x80x9d, Nature (London), Vol. 403, 750-753, 2000). Emission of light from DCM2 is the emission of light (fluorescent light) from the singlet excited state, and offers an advantage in that the triplet excited energy efficiently generated from the iridium complex can be utilized as the singlet excited energy of DCM2 of other molecules.
As described in literatures 2 to 4, the organic EL element capable of converting the triplet excited energy into light, makes it possible to accomplish an external quantum efficiency higher than convention alone. The luminous intensity increases with an increase in the external quantum efficiency. It is therefore considered that the organic EL element capable of converting the triplet excited energy into light will occupy an increasing weight in the future development as means for accomplishing the emission of light of high brightness and high light emission efficiency.
However, platinum and iridium are both so-called noble metals. Therefore, the platinum complex and the iridium complex using them are expensive and, it is expected that they pose barrier against lowering the cost in the future. In addition, if it is taken into consideration the effect of the metal complex containing heavy metals upon the human body, it is desired to use safer and easily disposable material.
The color of light emitted by the iridium complex is green, i.e., has a wavelength located in the middle of the visible light region. There has not been reported light of any other color emitted by the metal complex using iridium. Further, when the platinum complex is used as a dopant, light that is emitted exhibits red color of a relatively good purity. When the concentration is low, however, the host material, too, shines causing the color purity to be deteriorated. When the concentration is high, however, the light-emitting efficiency decreases due to the concentration quenching.
Namely, highly efficient emission of light of red and blue colors of high purities is not obtained from the organic EL elements capable of converting the triplet excited energy into light. From the standpoint of fabricating a flat panel display of full colors by emitting light of red, green and blue colors, therefore, light of red color and blue color of high purities must be emitted by using a cheap material yet maintaining a high external quantum efficiency like the iridium complex or the platinum complex.
Under the above-mentioned circumstances, it has been urged to develop an organic compound capable of converting the triplet excited energy into light (i.e., capable of emitting phosphorescent light) in addition to the existing iridium complex and the platinum complex.
It is a subject of the present invention to provide an organic compound capable of converting the triplet excited energy into light cheaply than the conventional compounds. It is further another subject of the invention to provide an organic EL element which features a high light-emitting efficiency and which can be cheaply formed by using the organic compound.
Further, it is a subject of the invention to provide a light-emitting device which is fabricated by using an organic EL element of a high light-emitting efficiency obtained by the invention, which is bright consuming a decreased amount of electric power, and which is cheaply constructed, as well as to provide an electric appliance using the light-emitting device.
The present inventors have given attention to the effect of heavy atoms that has been known in the field of photoluminescence (hereinafter referred to as xe2x80x9cPLxe2x80x9d). The effect of heavy atoms stands for the effect in which the spin-orbit interaction increases as heavy atoms (holding many atomic nucleus loads) are introduced into the molecules or into the solvent promoting the emission of phosphorescent light. Here, the atomic nucleus load corresponds to the atomic number, i.e., corresponds to the number of positive electric charges of the nucleus.
In order to convert the triplet excited energy into light, therefore, the present inventors have considered that it is most important to introduce the molecular structure having a large spin-orbit interaction. That is, the inventors have considered that the triplet excited energy can be converted into light by introducing the molecular structure having a large spin-orbit interaction even without using heavy atoms, in addition to introducing the action of heavy atoms by using heavy atoms.
As one method, it can be considered to introduce a molecular structure exhibiting ferromagnetic property or diaferromagnetic property. However, neither the metal complexes nor the organic coloring matters used in the conventional organic EL material have a molecular structure that exhibits such a property. It therefore becomes necessary to employ an organic compound having a molecular structure different from that of the material that has heretofore been used for the organic EL elements, so as to exhibit ferromagnetic property or diaferromagnetic property.
The present inventors, therefore, have given attention to a binuclear complex (metal complex having two central metals). The reason is because, the binuclear complex having paramagnetic metal ions often exhibits ferromagnetic or diaferromagnetic interaction in the complex.
In a binuclear complex (hereinafter referred to as xe2x80x9ccluster complexxe2x80x9d) in which the two central metals are metal-to metal bonded together (hereinafter referred to as xe2x80x9cMxe2x80x94M bondxe2x80x9d), in particular, the present inventors consider that the total nuclear load further increases and may trigger an effect which is substantially the same as the effect of heavy atoms. This also accounts for why the inventors have given attention to the binuclear complex.
The Mxe2x80x94M bond is easily formed under a condition where the d-orbits of the two metal atoms in the metal complex have close energy levels, are expanding sufficiently broadly and have suitable shapes. It is further important that the arrangement of electrons of the orbits is suited for the Mxe2x80x94M bond.
Here, as for the expansion of the d-orbit, the element located at a left portion of elements of the transition series is advantageous, and the element positioned on the lower side is more advantageous, for the Mxe2x80x94M bond. Therefore, niobium and tantalum (Group V), molybdenum and tungsten (Group VI), and technetium and rhenium (Group VII) easily form the Mxe2x80x94M bond in the metal complex. In particular, technetium, rhenium, molybdenum and tungsten exhibit strong bonding forces, often forming a quadruple bond in the metal complex.
Among them, molybdenum and tungsten which are elements of the Group VI are cheaply available metals and are suited for the present invention. Chromium which is an element of the Group VI and is located in the uppermost part of periodic table, has the electron arrangement of the d-orbit like those of molybdenum and tungsten, and is considered to be suited for the Mxe2x80x94M bond.
From the foregoing, this invention has a feature in that a cluster complex (hereinafter referred to as xe2x80x9ccluster complex of the Group VIxe2x80x9d) having an element of the Group VI (chromium, molybdenum, tungsten) as a central metal is used for the organic EL element as an organic compound for increasing the spin-orbit interaction.
Here, in particular, tungsten is a heavy atom comparable to iridium or platinum (the atomic number of tungsten is 74 which is nearly comparable to the atomic number 77 of iridium or to the atomic number 78 of platinum), and is highly probable to exhibit the effect of heavy atoms like those of iridium or platinum. Among the above-mentioned cluster complexes of the Group VI, therefore, it is expected that tungsten promote the emission of phosphorescent light relying not only upon the effect of Mxe2x80x94M bond but also upon the effect of heavy metals.
As heavy metals comparable to iridium and platinum, further, there can be exemplified transition metals pertaining to the elements of the third transition system. Among them, however, tungsten exists in the greatest amount on earth and is cheaply available, and is suited for this invention.
Therefore, this invention has a feature of using, among the cluster complexes of the Group VI, the metal complex (hereinafter referred to as xe2x80x9ctungsten complexxe2x80x9d) having tungsten as a central metal for the organic EL element as the organic compound in order to increase the spin-orbit interaction.
It is relatively easy to prepare the cluster complex of the Group VI by changing only the central metal in the same ligand. It is considered that the excited energy state changes depending upon the central metal. It is therefore expected to change the color of the emitted light by changing the central metal, creating a large feature of the cluster complex of the Group VI.